This invention relates generally to multi-stand metal rolling mills and, more particularly, to a method of improved control of strip thickness in such mills.
Modern multi-stand cold rolling mills commonly employ a form of feed-forward gage control which acts between adjacent stands of such mills. A thickness gage situated between these stands measures strip thickness and sends a strip thickness data signal, with some inherent time delay, from the thickness gage to the downstream rolling stand. Upon arrival at the downstream stand a control action is initiated, most commonly an adjustment to the upstream speed reference in proportion to the thickness change from an initial measurement, or from some nominal thickness. For example, if a strip's thickness increases by one percent from its initial thickness, then upon arrival of the thicker strip region at the downstream stand a one percent reduction of the initial upstream stand speed reference would be made. The resulting decrease in upstream stand speed would cause an increase in interstand tension which would reduce the downstream stand rolling force and, accordingly, the gage exiting the downstream stand. In another common control arrangement, tension between adjacent stands is controlled by adjustment of the roll gap of the downstream stand. There the roll gap is closed to restore tension to some reference level, so as to reduce the downstream stand exit gage by an amount more nearly proportional to the upstream stand speed change.
An objective of feed-forward control is to improve the uniformity of strip thickness out of the downstream stand. Control of the absolute strip thickness is the objective of later control action, such as feedback control based on final thickness measurements, and is not the subject of this invention.
These control strategies and operating practices are well known and have been thoroughly described in the rolling literature, for example "Thickness Control in Cold Rolling" by D. J. Fapiano and D. E. Steeper, Iron and Steel Engineer, November 1983, and "New Approaches to Cold Mill Gage Control" by W. D. King and R. M. Sills, AISE Yearly Proceedings, 1973, p. 187.
There are two principal weaknesses in prior art embodiments of the feed forward gage control strategy. First, upstream stand speed changes are delayed by the response time of the stand speed regulator. Considerable design effort by others has been directed to improvement of this response time by various forcing functions. Second, assumptions are made that the strip thickness exiting the downstream stand will be uniform if the upstream stand speed is adjusted in proportion to the changes in strip thickness entering the downstream stand. This assumption is not sufficient if the downstream stand speed changes, as a result of a change in interstand tension produced by the change in upstream stand speed. In older mills employing screw type gap controls, the tension control by gap adjustment is relatively slow and results in tension changes which produce transient load disturbances on both adjacent stands. These, in turn, produce speed changes that oppose the desired change in relationship between the speeds of the adjacent stands. The temporary tension increase produced by lowering the upstream stand speed would cause downstream stand speed to fall so that the resulting gage correction would be less than desired. While this problem might be addressed by some form of approximate compensation, the required compensation would depend on schedule dependent factors which are difficult to model, and in fact, have not been included in known feed-forward control systems.
To overcome the deficiencies of the prior methods, it is desirable to determine and apply the feed-forward corrections in a manner which accounts for the speeds of both adjacent stands, and also to improve the responsiveness of the main drive control to the feed-forward corrections.
It is, therefore, an objective of the present invention to provide an improved method of rolling metal strip.
It is a further objective to provide a method of strip thickness control which reduces strip thickness variations caused by inadequate response of main drive speed regulators and by unplanned variations in rolling mill speeds.